Dynamic Paint Cycles - How do you make waves caused by an object to turn white?
Geometry Input - Pointiness

You might want to experiment with the Geometry Input and try out Pointiness to mix the whitecaps with the water.





Update


The order of the Shader inputs is important. If white becomes the dominant color, the inputs on the Mix Shader need to be swapped.
The black value in the color ramp should start at 0.5 or above. We want to preserve the water material and only add the whitecap material to the pointy parts of the mesh. 


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Here's your updated blend file: